dt-bindings: reserved-memory: Introduce memory-region-names

In order to make the reserved-memory bindings more consistent with other
existing bindings, add a memory-region-names property that contains an
array of strings that name the entries of the memory-region property and
allows these regions to be looked up by name.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>